1. Field of the Invention
This invention relates to laser beam projection systems utilizing controllable reflectors to compensate for medium-aberrated waves and, more particularly, to such systems utilizing an additional controllable reflector to compensate for aberrations inherent in the principal controllable reflector.
2. Background Information
Since the invention of the laser in the 1950s, a great number of applications have been developed for exploiting the spectral purity and spatial coherence of the laser beam. Communications, data transfer, and the projection and processing of images have come to depend upon the unique properties of the coherent laser wavefront. These properties must be preserved if the powerful and beneficial qualities of laser radiation are to be fully utilized. Except in free space, a laser beam travels through a material medium and the wavefront quality of the laser beam is reduced: high spatial quality waves become aberrated; plane waves emerge with randomly perturbed wavefronts. The diffraction associated with such aberrated waves significantly reduces the ability to focus the beam to a high-quality beamspot or to efficiently transmit a communications signal to a remote receiver.
Another problem occurs when such laser wavefronts are transmitting large amounts of energy. Some portion of that energy is absorbed when it passes through a given optics system of lenses, mirrors, and other optical devices, or when it travels through the atmosphere. Typically, when materials absorb energy and heat up, their index of refraction changes. This change in index varies across a given beam profile. The intensity of the beam and the amount of heat absorbed vary as a function of location within the beam. Differences in index cause refraction of a laser beam. The consequent spreading of the high-energy laser beam due to a laser-induced index differential is termed "thermal blooming". Due to thermal blooming, the beam which arrives at its target has spread too far, and even if it is focused, the phase of the beam across its wavefront is so randomized that on the whole it destructively interferes and cancels itself out. Thus the laser beam delivers only a small fraction of the energy being transmitted to the receiving site.
In order to counteract these deleterious effects of atmospheric turbulence, thermal blooming, and irregularities within the optical train, adaptive optical systems have been explored and developed. These systems combine wavefront sensing and wavefront correction within a closed feedback loop in order to correct a particular laser beam's wavefront errors. A typical laser beam direction system might work as follows. A laser beam is directed via an atmospheric path to a target or receiving site. Because of turbulence and thermal blooming, only a portion of the radiation reaches the target. In some systems, a laser reference is transmitted back through the atmosphere in order to be used as a probe wave which samples the atmospheric aberrations the light has encountered. In essence, the return signal contains in its wavefront phase all the aberrations of the beam path. If the phase aberrations are then sensed and the laser beam is pre-aberrated to correspond to this phase pattern, during its propagation through the atmosphere, the laser will retrace the path of the target radiation and arrive at the target unaberrated; the full amount of beam energy will then have been transferred.
A variety of apparatus and methods have been developed or proposed for this type of beam correction. These include deformable mirror systems and phase conjugation approaches using nonlinear optical media.
Deformable mirror systems suffer from a great number of inherent problems. The use of discrete, bulky electrical actuators limits the spatial frequency response for the mirror; a deformable mirror simply cannot correct errors finer than the spacing of the push-pull actuator elements. In addition, such actuators typically require several thousand volts for operation and are subject to arc-overs and permanent breakdowns. Their impedance combined with the mass of the mirror surface limits the temporal frequency response of the adaptive system. Each detector/actuator feedback loop requires discrete electronic processing systems and considerable amplification to function properly. Since the thin front surface of the mirror continuously experiences flexures, it suffers from eventual drift and creep problems with consequent loss in performance.
Nonlinear optical media, using degenerate four-wave mixing, stimulated Brillouin or Raman scattering, can provide a time-reversed wave as an output in some applications. While this phase-conjugation method greatly improves the spatial resolution of the system since the "actuator spacing" is now molecular, it also presents certain problems. The input sensitivity is very low, so that a fairly large return signal is necessary in order to set up the proper index grating structure within the phase-conjugation cell. Such power requirements rule out this method for lower-power optical communication and data transfer systems. In addition, the pump beams require an enormous amount of energy and must be precisely aligned for the device to function. The wasted costs of duplicate high-energy lasers for pumping the medium and the resulting low efficiency conversion and transmission of the energy to the target make phase conjugation an interesting but often impracticable means for adaptive optics applications.
An alternative approach to the phase-conjugation method mentioned above involves the use of a liquid crystal light valve incorporating a self-adaptive control system to combine both wavefront sensing and wavefront correction in a single package. Boswell et al in U.S. Pat. No. 4,019,807 disclose one particular version of a standard liquid crystal light valve (LCLV). If the usual liquid crystal element in that disclosed device is replaced with a parallel-aligned tunable birefringent liquid crystal substrate, the modified LCLV device can be used as a deformable mirror. It thus finds particular utility in the wavefront sensing and correction system. A number of benefits flow from the use of such an LCLV in this type of system. The LCLV itself requires no signal processing, electric amplification, or high-voltage sources. It possesses a spatial resolution of approximately 30 microns per pixel, far better than the typical deformable mirror which may have 16 to 60 pixels spread over an area of 100 square centimeters.
In such a system, aberrated light from the target, degraded by all the distortions of its travel path, passes through the substituted liquid crystal element, reflects off a dielectric mirror, passes through the liquid crystal element once more, and is transferred by various optical means to the back side or photoconductor side of the LCLV where it combines with a locally generated unaberrated wavefront. The two combined wavefronts create an interference pattern which through careful alignment is in exact registration with the incident target wavefront hitting the front side of the LCLV. A photoconductive layer lies sandwiched behind the liquid crystal element between the dielectric mirror and one clear conductive substrate. Once a voltage is placed across these conductors, any change in resistance in the photoconductor due to absorbed radiant energy engenders a commensurate change in voltage across the liquid crystal element at that point. The interference pattern impinging on the photoconductive layer provides such a source of radiant energy and the pattern represents an exact spatial mapping of the wavefront errors of the incoming target radiation. Hence, voltages across the liquid crystal element will change at precisely those points where the phasefront of the target radiation is aberrated. The refractive index and hence optical path length of the liquid crystals will change at those points due to the applied field. Thus, these microscopic phase-shifting elements will "push or pull" the incoming wavefront until a uniform interference pattern is obtained upon the photoconductive side of the liquid crystal light valve device, at which point the servosystem is in equilibrium.
When the interference pattern is uniform, the incoming wavefront is completely corrected and the liquid crystal "mirror" can be used to reflect a pre-aberrated high energy laser beam so that it arrives at a given target point completely unaberrated. Since the LCLV typically requires very little power to produce the desired phase modulation effects, the element can also be used to great advantage in very precise image and data processing systems. Moreover, the rather uniform wavelength dependence of the phase shifts created in the liquid crystal mirror allows multiple wavelength use of the device, just as in conventional deformable mirror technology.
Modified silicon liquid crystal valves (LCLVs) have been selected for use in the system described above for adaptively correcting for atmospheric aberration because of the relatively quick response of the thin silicon device. However, the modified silicon LCLV presents one disadvantage in that it exhibits an inherently high optical non-uniformity. Methods to improve the output uniformity of such systems have been proposed, utilizing external fixed correction plates. However, these plates, which must be made separately for each light valve, are difficult to manufacture and furthermore they cannot be adapted to changes in the light valve structure which may occur over long periods of time. A more effective way of compensating for the optical non-uniformity of the modified silicon LCLV is needed if systems incorporating the LCLV are to reach their full potential.